Peel the sweet potato and apple, then chop them into small, even 1/2-inch pieces. Smaller, uniform pieces cook faster and more evenly, which helps you get a silky-smooth puree with less blending.

Place the chopped sweet potato and apple in a small saucepan. Add just enough water to cover the bottom of the pan by about 1/2 inch. Cover with a lid and bring to a gentle simmer over medium-low heat. Cook for 10–15 minutes, stirring once or twice, until both are very tender and easily mashed with the back of a spoon.

Turn off the heat. Carefully pour off any excess cooking liquid into a cup or bowl and set aside. You will use this flavorful liquid to thin the puree, keeping nutrients in the food instead of discarding them.

Transfer the cooked sweet potato and apple to a blender, food processor, or blend directly in the pot using an immersion blender. Add about 1/4 cup of the reserved cooking liquid, breast milk, or prepared formula. Blend on low, then increase the speed gradually until the mixture is completely smooth with no visible lumps, scraping down the sides as needed.

For very young babies or first-time feeders, aim for a thin, runny-yogurt consistency. Add more of the reserved liquid, breast milk, or formula 1 tablespoon at a time, blending after each addition until you reach the desired texture. For older babies, you can leave the puree slightly thicker or with very tiny, soft bits.

If your baby is at least 6 months old and already tolerates this puree well, blend in 1–2 teaspoons of unsalted butter or neutral oil and, if desired, a tiny pinch of ground cinnamon. Introduce only one new ingredient at a time so you can watch for any reactions.

Transfer the puree to clean, shallow containers or baby food storage trays. Let it cool completely to room temperature before serving or refrigerating, as hot spots can linger in thick purees. Serve a small portion immediately if desired and store the remainder in the fridge or freezer.